The NOνA far detector, at Ash River Minnesota, measures neutrinos fired from Fermilab in Chicago – 800 km away. This week NOνA reported data showing that they change types during that journey; the beginning of what promises to be an exciting programme of precision neutrino physics. And meanwhile, in Antarctica…
I’m writing this in a coffee bar in Chicago. The ‘windy city’ seems quiet and still this morning and the coffee is surprisingly good. About 80 km to the west, at Fermilab, the highest-powered beam of neutrinos in the world is being produced, and fired through hundreds of kilometres of solid rock to impatiently-waiting detectors, principally the new NOνA far detector.
The reason for my visit is the annual Boost meeting, more related to the Large Hadron Collider than to neutrinos. Fermilab hosted the previous highest-energy particle-collider, the Tevatron, and there is a big community of high-energy physicists in and around Chicago who now work on the LHC. Some of them are hosting us this week. But these days neutrinos are the focus of the accelerator programme down the road¹.
Neutrinos are made by firing protons into a target. This produces lots of mess, including charged particles called pions (made of a quark and an antiquark), which travel a while and can be focussed into a beam. They eventually decay to neutrinos, which remain in a collimated beam and, mostly, just carry on without interacting with anything. Crucially though, a few of them will, by random luck (maybe bad luck from the neutrinos’ point of view) collide with normal matter, some of it (by good luck from the physicists’ point of view) the matter inside the NOνA far detector, which can measure what kind of neutrinos they were.
The vast majority of neutrinos produced when a pion decays are so-called “muon neutrinos”. This means when they interact they should produce muons (a heavier version of the electron). If the neutrinos did nothing odd during their 800 km journey to NOνA, about 200 of them should have been seen by now. However, only 33 turned up. Also, six electron neutrinos turned up, when only about one would be expected.
This is evidence that the neutrinos transmogrify, or ‘oscillate’, during their journey. That is, they change types. This behaviour is already known; it is how we know neutrinos have mass (in the original version of the Standard Model of particle physics they were massless), and it may be connected with mysterious fact that there is so much matter around and so little antimatter. Studying this kind of mystery is what Noνa was built for, and this confirmation of neutrino oscillations is just the start. There is competition and complementarity with SuperKamiokande / T2K in Japan, and the NOνA physicists are glad of this strong start. As NOvA co-spokesperson Mark Messier of Indiana University said
Having a beam of that power running so efficiently gives us a real competitive edge and allows us to gather data quickly.
Meanwhile, down South
The power and intensity of the Fermilab neutrino beam is impressive, but nature still trumps us when it comes to energy. Also this week, the Ice Cube detector in the antarctic saw the highest energy neutrino ever recorded.
It was a muon neutrino. That kind of neutrino is especially interesting for astrophysics, because the muon which is produced when it hits the ice points back in the direction from which the neutrino came. Since the neutrino itself travels to us unpertrubed by galactic magnetic fields or interstellar gas, that should give a rather accurate pointer to its source – presumably some violent astrophysical event. If, as Francis Halzen expects, similar events start being reported by Ice Cube about once a month now, this is really the opening of a new window on the universe.
Declaration of interest – UCL have just joined NOνA, though that’s really not why I am writing this.
¹Fermilab is in the Chicago suburb of Batavia. The neighbouring suburb is Geneva, Illinois. The means that the current and previous high-energy record-holding machines were built next to a Geneva. Rumours that part of China is to be renamed have just started.
Also at the Guardian.